1. Field of the Invention
This invention relates to a device for measuring dielectric constant and conductance.
The composition of a material may often be characterised in terms of its dielectric constant or conductance. For example moisture content can often be correlated to the dielectric constant, while in solutions of ionic salts conductivity measurements are sometimes used to measure the concentration. Instruments which measure dielectric constant usually operate at radio frequencies where the capacitive reactance can more easily be measured.
It is an object of this invention to provide an instrument or device which is capable of providing direct measurements of both dielectric constant and conductance or alternatively may be calibrated to provide direct measurements of liquid composition where the relationship between liquid composition and dielectric constant and conductivity is unknown. A further object of the invention is to provide a device capable of a direct measurement of two components in solution; for example to provide direct measurements of both sucrose and ash and ionic salts in a solution containing both these substances.
The device of the invention has potential application in the sugar industry for the measurement or control of brix of liquors or massequites during pan boiling or measurement of ash in sugar refinery liquors.
2. Description of Related Art
The instrument also has general application in the chemical industry for measurement of moisture or solids on a variety of materials.
An EPO search revealed several patents as follows:
Dealing with these patents:
U.S. Pat. No. 5,199,671
This concerns a method for measuring the composition of a mixture of different fuels having different dielectric constants. The frequency of a variable frequency oscillator is determined when immersed but insulated from the fuel to be measured. The difference gives a measure of the dielectric constant of the fuels.
However, the difficulty of constructing an oscillator which is stable to temperature differences is well known so that if the temperature of either or any of the fuel changes, the measurements will be inaccurate. This inaccuracy is overcome in the present invention by using an electrode as the measuring element and having reference oscillator frequency measurements with the electrode in and out of circuit. In addition the present invention measures frequency shift at two oscillator frequencies allowing the composition to be related to complex impedances (dielectric constant and resistance).
U.S. Pat. No. 4,426,616
In this patent the capacitance of a probe is measured where there is parallel resistance across the capacitor. Fixed frequency generators are used as compared with the variable frequency oscillators of the present invention. The measurements are also of an analog nature as opposed to the present invention in which they are digital.
Publications
MINIATURE-ELECTRODE PROBE FOR CONDUCTIVITY MEASUREMENTS ON ELECTROLYTIC SOLUTIONS OR COLLOIDAL SOLS OF SMALL SAMPLE SIZES.
This instrument described is a miniature electrode for use where the quantity of sample is small (eg blood samples )
Measurement of conductivity is carried out at audio frequencies (10 kHz) using a system similar to that conventionally used for conductivity measurement.
MEASURING METHOD FOR ELECTROSTATIC CAPACITY
Appln. No. 60-75159
This invention involves a variable frequency oscillator for the measurement of capacitance where the unknown capacitor plus a known capacitor are alternatively switched into the oscillator circuit by a change over switch and the two frequencies measured.
From the two frequency measurements the value of the unknown capacitor can be calculated using a prescribed formula.
This apparatus is only useful for measuring capacitance provided there is no resistance in the measuring element.
PCTAU91/00467
This invention measures moisture and salinity.
Two electrodes are connected across an inductance to form a tank circuit of a variable frequency oscillator where the oscillator frequency varies according to the physical properties of the material between and surrounding the electrodes.
There are two modes of operation, one where a certain value of inductor is switched into circuit which results in the oscillator operating at a frequency of around 150 MHz. At this frequency, the shift in oscillator frequency due to varying soil conductivity is small in comparison with that caused by changing soil moisture (resulting in changing dielectric constant). This measurement is therefore representative of soil moisture.
In the second mode of operation, the inductor is replaced by a larger value inductor such that the frequency of oscillation is reduced to about 10 MHz. At this frequency the oscillator frequency is affected by both soil conductivity and moisture content (dielectric constant). By measuring the oscillator frequencies at both modes and calculating a difference, it is claimed that a signal representative of conductivity can be derived.
The differences between this and the present invention are as follows:
i) In the above invention, the electrodes are connected directly across the inductor. This will result in a fairly large frequency shift due to changing soil conditions, but this method will not work if the measurement system is used in a medium of high conductivity (such as low purity massecuites in sugar pans) since the Q of the circuit will be reduced (excessive damping) to a stage where the oscillator will not oscillate). The system will, however, probably work for measurement of moisture in cotton, grain, coal dust and concrete a claimed since these materials are not very conductive. PA1 ii) It is difficult to construct a variable frequency oscillator that is stable over a wide temperature range as is experienced in an industrial measurement environment. Temperatures of the measurement instrument electronics can typically vary from 10.degree. C. to 70.degree. C.
To prevent loss of oscillation with highly conductive media, it is necessary to use a capacitive divider or some other means, to reduce coupling of the measurement electrodes to the tuned circuit. This, however, reduces the frequency shift of the oscillator due to material surrounding the electrodes.
The above instrument relies on an oscillator which is stable and only two frequencies (corresponding to the two modes) is measured. Any shift in oscillator frequency due to temperature variations (of the oscillator) will result in inaccuracy. Because of the high degree of coupling, the frequency shifts in the designed application are high, and drift in oscillator frequency may not introduce too significant an error. The degree of accuracy required for agricultural purposes is also not as high as that required for an industrial measurement instruments to be used for control.
In the present invention, the effects of oscillator drift are compensated for by switching the probe in and out of circuit and taking two measurements; a reference frequency measurement with the probe out of circuit, and a second measurement with the probe in circuit. The difference between these frequencies is used as the measurement. Since these two measurements are taken in succession (within a 2 second interval) and both measurements are subject to a similar drift, the effect of this drift is greatly reduced thereby increasing instrument accuracy.
A second set of measurements (measurement plus reference) is taken at a frequency about 10 MHz different to the first.
EUROPEAN PATENT NO. 0 472 767 A1
Vorrichtung zum Feststellen des Alkoholhaltes oder des Heizwertes eines Gemichs.
This patent describes a device used for measuring the alcohol content of fuel mixtures by measuring the dielectric constant conductance, and temperature of the mixture and then using these results to compute the alcohol content or calorific value of the fuel.
The patent covers the measuring cell which is described in detail and basically consists of two electrodes and a temperature sensor mounted in a housing through which the fuel is passed.
The capacitance is measured between the first electrode and the housing, and the conductance measured by the second electrode. A temperature sensor is installed in the first electrode.
The method of measuring the capacitance and conductance is not described in detail except that measurements are carried out at 10 MHz and that measurement takes 100 m secs.
The electronics are mounted in the probe assembly.
There appears no similarity between this invention and the present invention in which measurement is between two electrodes in the medium to be measured. The measurement system in the prior invention apparently only measures at one frequency.